Just my tuppence, getting back to Ratmors's OP.
If it's flaking off then it sounds to me like it was a waxoyl or similar spray on decoration job over an aleady rusting chassis without any decent preparation work before coating..
What i would do is get the whole underbody steam cleaned and see what the chassis is like and then decide how to go about good preventative treatment, if its flaking then arguably the present muck needs to come off completely because it will be holding salt water in places where it's peeling which can't drain or dry out and the chassis would be better off naked as it were for the rest of the winter.
Depending on condition and what you wish to spend, it might be better to spray the whole chassis in whatever products one prefers once cleaned off, i use ACF50 regularly, some recommend Krown, others would spray chain lube liberally about, an old chap i spoke to regularly swore by Castrol CL waterproof grease which seems to have vanished but similar marine use greases are available quite cheaply via ebay, circa £40 for a 12.5kg tub, that would coat a lot of chassis by hand painting on and chain lube or whatever sprayed on after to seep into the nooks, once you go down this route it's going to be even harder to revert to painting, so if you go oily products you are going to have to keep that method up permanently.
A full professional treatment able to withstand future pressure washing is going to cost big money, starting at £1000, it costs around £450 for a pro waxoyling, i know this because i had the job done to my then new Hilux which needed no prep work at all not even a wash, and to be perfectly honest i can and do a better job myself with better products.
You are wanting a hard finish, that means full descaling of the chassis, prep treatments and full coverage with some serious paints and probably baked hard, i dare this is possible to find but will take a long time and will cost.
Unless you are prepared for a week or more's worth of work and a bill well into 4 figures, it might be just as good long term to get it basically treated then coated with one of the oil/lanolin based products but accept that this will need inspection and additional coverage, every year ideally.
Remember it isn't just the chassis that needs cavity spraying with something oily that creeps, the body sections need this too.
We shouldn't need these conversations, Toyota should by the time the 120/100/200 were released have gotten round to zinc plating the chassis and suspension/drivetrain castings for countries that see salt in ridiculous quantities like ours.